The accessories in a conventional motor vehicle are powered by output from the engine. A hybrid electro-mechanical vehicle generally includes both an internal combustion engine and one or more electric motor/generators. Some of the accessories in a hybrid vehicle may require power while the engine is off and the vehicle is at a stand-still or being powered by the electric motor/generators. Traditionally, in order to power hybrid vehicle accessories when the engine is off, it was necessary to provide each such accessory with a separate electric motor.
The typical automotive accessory drive system consists of a drive pulley connected to an output shaft of the engine, typically the crankshaft. Wrapped around this pulley is a flexible drive belt, which in turn is wrapped around a plurality of driven pulleys. This flexible drive belt transmits drive forces between the drive pulley and the driven pulleys. The driven pulleys may be drivably attached to accessories known in the art such as a power steering pump, air conditioning compressor, alternator, and water pump.
An electrically variable transmission (EVT) utilizes one or more electric devices, such as motors/generators, and a differential gearing arrangement to provide a continuously variable ratio drive between input and output. An EVT is particularly useful in hybrid electric vehicle powertrains including an engine that is directly coupled to the transmission input and also including a storage device, such as a battery, used to supply power for propulsion and to recover energy from braking.